GDB: find line where function exits [duplicate] - gdb

Question

I have a C++ function which has many return statements at various places. How to set a breakpoint at the return statement where the function actually returns ?
And what does "break" command without argument means?

Answer 1

Contrary to answers so far, most compilers will create a single return assembly instruction, regardless of how many return statements are in the function (it is convenient for the compiler to do that, so there is only a single place to perform all the stack frame cleanup).
If you wanted to stop on that instruction, all you have to do is disas and look for retq (or whatever the return instruction for your processor is), and set a breakpoint on it. For example:
int foo(int x)
{
switch(x) {
case 1: return 2;
case 2: return 3;
default: return 42;
}
}
int main()
{
return foo(0);
}
(gdb) disas foo
Dump of assembler code for function foo:
0x0000000000400448 <+0>: push %rbp
0x0000000000400449 <+1>: mov %rsp,%rbp
0x000000000040044c <+4>: mov %edi,-0x4(%rbp)
0x000000000040044f <+7>: mov -0x4(%rbp),%eax
0x0000000000400452 <+10>: mov %eax,-0xc(%rbp)
0x0000000000400455 <+13>: cmpl $0x1,-0xc(%rbp)
0x0000000000400459 <+17>: je 0x400463 <foo+27>
0x000000000040045b <+19>: cmpl $0x2,-0xc(%rbp)
0x000000000040045f <+23>: je 0x40046c <foo+36>
0x0000000000400461 <+25>: jmp 0x400475 <foo+45>
0x0000000000400463 <+27>: movl $0x2,-0x8(%rbp)
0x000000000040046a <+34>: jmp 0x40047c <foo+52>
0x000000000040046c <+36>: movl $0x3,-0x8(%rbp)
0x0000000000400473 <+43>: jmp 0x40047c <foo+52>
0x0000000000400475 <+45>: movl $0x2a,-0x8(%rbp)
0x000000000040047c <+52>: mov -0x8(%rbp),%eax
0x000000000040047f <+55>: leaveq
0x0000000000400480 <+56>: retq
End of assembler dump.
(gdb) b *0x0000000000400480
Breakpoint 1 at 0x400480
(gdb) r
Breakpoint 1, 0x0000000000400480 in foo ()
(gdb) p $rax
$1 = 42

Answer 2

You can use reverse debugging to find out where function actually returns. Finish executing current frame, do reverse-step and then you should stop at just returned statement.
(gdb) record
(gdb) fin
(gdb) reverse-step

Answer 3

Break on all retq of current function
This Python command puts a breakpoint on every retq instruction of the current function:
class BreakReturn(gdb.Command):
def __init__(self):
super().__init__(
'break-return',
gdb.COMMAND_RUNNING,
gdb.COMPLETE_NONE,
False
)
def invoke(self, arg, from_tty):
frame = gdb.selected_frame()
# TODO make this work if there is no debugging information, where .block() fails.
block = frame.block()
# Find the function block in case we are in an inner block.
while block:
if block.function:
break
block = block.superblock
start = block.start
end = block.end
arch = frame.architecture()
pc = gdb.selected_frame().pc()
instructions = arch.disassemble(start, end - 1)
for instruction in instructions:
if instruction['asm'].startswith('retq '):
gdb.Breakpoint('*{}'.format(instruction['addr']))
BreakReturn()
Source it with:
source gdb.py
and use the command as:
break-return
continue
You should now be at retq.
Step until retq
Just for fun, another implementation that stops when a retq is found (less efficient of because no hardware support):
class ContinueReturn(gdb.Command):
def __init__(self):
super().__init__(
'continue-return',
gdb.COMMAND_RUNNING,
gdb.COMPLETE_NONE,
False
)
def invoke(self, arg, from_tty):
thread = gdb.inferiors()[0].threads()[0]
while thread.is_valid():
gdb.execute('ni', to_string=True)
frame = gdb.selected_frame()
arch = frame.architecture()
pc = gdb.selected_frame().pc()
instruction = arch.disassemble(pc)[0]['asm']
if instruction.startswith('retq '):
break
ContinueReturn()
This will ignore your other breakpoints. TODO: can be avoided?
Not sure if it is faster or slower than reverse-step.
A version that stops at a given opcode can be found at: https://stackoverflow.com/a/31249378/895245

Answer 4

break without arguments stops execution at the next instruction in the currently selected stack frame. You select strack frames via the frame or up and down commands. If you want to debug the point where you are actually leaving the current function, select the next outer frame and break there.

Answer 5

rr reverse debugging
Similar to GDB record mentioned at https://stackoverflow.com/a/3649698/895245 , but much more functional as of GDB 7.11 vs rr 4.1.0 in Ubuntu 16.04.
Notably, it deals with AVX correctly:
gdb reverse debugging fails with "Process record does not support instruction 0xf0d at address"
"target record-full" in gdb makes "n" command fail on printf with "Process record does not support instruction 0xc5 at address 0x7ffff7dee6e7"?
which prevents it from working with the default standard library calls.
Install Ubuntu 16.04:
sudo apt-get install rr linux-tools-common linux-tools-generic linux-cloud-tools-generic
sudo cpupower frequency-set -g performance
But also consider compiling from source to get the latest updates, it was not hard.
Test program:
int where_return(int i) {
if (i)
return 1;
else
return 0;
}
int main(void) {
where_return(0);
where_return(1);
}
compile and run:
gcc -O0 -ggdb3 -o reverse.out -std=c89 -Wextra reverse.c
rr record ./reverse.out
rr replay
Now you are left inside a GDB session, and you can properly reverse debug:
(rr) break main
Breakpoint 1 at 0x56057c458619: file a.c, line 9.
(rr) continue
Continuing.
Breakpoint 1, main () at a.c:9
9 where_return(0);
(rr) step
where_return (i=0) at a.c:2
2 if (i)
(rr) finish
Run till exit from #0 where_return (i=0) at a.c:2
main () at a.c:10
10 where_return(1);
Value returned is $1 = 0
(rr) reverse-step
where_return (i=0) at a.c:6
6 }
(rr) reverse-step
5 return 0;
We are now on the correct return line.

Answer 6

If you can change the source code, you might use some dirty trick with the preprocessor:
void on_return() {
}
#define return return on_return(), /* If the function has a return value != void */
#define return return on_return() /* If the function has a return value == void */
/* <<<-- Insert your function here -->>> */
#undef return
Then set a breakpoint to on_return and go one frame up.
Attention: This will not work, if a function does not return via a return statement. So ensure, that it's last line is a return.
Example (shamelessly copied from C code, but will work also in C++):
#include <stdio.h>
/* Dummy function to place the breakpoint */
void on_return(void) {
}
#define return return on_return()
void myfun1(int a) {
if (a > 10) return;
printf("<10\n");
return;
}
#undef return
#define return return on_return(),
int myfun2(int a) {
if (a < 0) return -1;
if (a > 0) return 1;
return 0;
}
#undef return
int main(void)
{
myfun1(1);
myfun2(2);
}
The first macro will change
return;
to
return on_return();
Which is valid, since on_return also returns void.
The second macro will change
return -1;
to
return on_return(), -1;
Which will call on_return() and then return -1 (thanks to the ,-operator).
This is a very dirty trick, but despite using backwards-stepping, it will work in multi-threaded environments and inlined functions, too.

Answer 7

Break without argument sets a breakpoint at the current line.
There is no way for a single breakpoint to catch all return paths. Either set a breakpoint at the caller immediately after it returns, or break at all return statements.
Since this is C++, I suppose you could create a local sentry object, and break on its destructor, though.